Physical oceanography

Walking on a Fine Line

What is a WireWalker? You might initially picture someone walking along a tightrope across some crazy abyss. From an observational oceanographer perspective, Wirewalkers are a type of marine robot we use to collect data in the ocean. Instead of walking horizontally, the Wirewalker traverses up and down a wire from the surface to the deep ocean.

The Art of WireWalking

The Wirewalker design is rather elegant because the up and down motion is entirely powered by wave energy – no need for fuel here! There is a buoy at the surface ocean, to which a long wire is tethered with a heavy weight on the end. In between a bright yellow platform holds all of the data collecting instruments and moves up and down along the wire. There is a neat video of the Wirewalker moving in the water that I recommend you check out.

Figure 1. The Wirewalker being tested in a tank at the Woods Hole Oceanographic Institution before deployment.

As the buoy bobs up and down in the waves, a downward force pushes the Wirewalker. After repeatedly being forced down the wire by the waves it eventually reaches the weight at the bottom of the wire. Hitting the weight switches a mechanism inside the device which frees it from downward motion. We put special foam inside the Wirewalker that can withstand large amounts of water pressure. This foam makes the platform buoyant, so when it is freed it will float back up to the surface along the wire. As it reaches the buoy this collision switches the mechanism back into one where it can feel the force of the waves and walk down the wire.

This design allows us to collect profiles of data at very rapid rates. Approximately every hour the Wirewalker is capable of traveling from the ocean surface to 500 m deep and then back to the surface. This is much faster than other robots, such as floats or gliders. Another great feature is that the Wirewalker can be left in the water detached from the ship following wherever the water takes it, which is called Lagrangian data sampling. If you only sample one location in the water you will be briefly sampling water as it passes by the instrument, but by moving the instrument with the water you can follow the same parcel of water to see how it changes with time.

Prepping for Deployment

As you may have read earlier this month, prepping to go to sea is a lot of work, but still very fun! I like to compare prepping the Wirewalker to building with giant legos. Each of the sensors is like a block that gets attached to the platform. Figure 2 shows a few stages in prepping the Wirewalker.

Figure 2. Attaching all of the sensors onto the WireWalker platform.

WireWalker Applications

What sensors are attached? Scientists can attach many different instruments to the base of the Wirewalker tailoring them to the experiment. Dr. Melissa Omand from the Graduate School of Oceanography at the University of Rhode Island recently deployed the Wirewalker as part of a NASA funded mission called EXPORTS (EXport Processes in the Ocean from Remote Sensing). You can track the Wirewalker’s location here as it moves about in the ocean! The goal of EXPORTS is to relate images NASA collects from satellites of the surface ocean to measurements collected by instruments deep in the water. This ultimately will help us better understand how we can observe and quantify the biological pump from space. The biological pump is when tiny plants in the ocean use carbon dioxide and sunlight to photosynthesize. When these organisms die or are eaten and eventually excreted, dense organic matter sinks and can be stored deep in the ocean for up to millenia!

On this mission, sensors on the Wirewalker are currently measuring the temperature and saltiness of the water, which is used to calculate the density. There is also an oxygen sensor, which will be used to estimate how much oxygen is produced by tiny plants called phytoplankton during photosynthesis. There are also different types of optical sensors that measure forms of light scattering in the water. Light emitted from the sensors is scattered by organisms and particles, which we can relate to the amount of carbon containing particles present. Collectively, these measurements along with a plethora of data collected during EXPORTS will help relate surface phytoplankton productivity detected by satellites to deep ocean carbon storage!

Acknowledgments

While the Wirewalker was originally developed by the Ocean Physics Group at Scripps Institution of Oceanography under sponsorship from the Office of Naval Research and the National Science Foundation, it is now produced commercially. A company called Del Mar Oceanographic now makes the robots and sells the robots. You can read more about the Wirewalker on their website.

I am a first year graduate student at the Graduate School of Oceanography at University of Rhode Island. I use robots and satellites to research the biological carbon pump, which is a series of processes that transfer carbon dioxide from the atmosphere into the deep ocean where the carbon is stored for long periods of time. I am particularly interested in the use of oxygen measurements to better understand how much carbon-containing material is produced by phytoplankton, tiny marine organisms, and is available for transport to the deep ocean. Learning about how much carbon the ocean stores through these processes is important to improve predictions about how climate change will impact the ocean.